1. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention provides a system and method for an IEEE 802.11 Access Point (AP) to prevent traffic suffering bad-link-quality from over occupying the channel and affecting other traffic. Most particularly, in the system and method of the present invention an AP identifies the traffic suffering from bad link quality and dynamically controls channel access to restrict the effect of bad-link-quality in an IEEE 802.11 local area network (LAN) environment.
2. Description of the Related Art
IEEE 802.11 LAN technology is increasingly popular and is becoming the dominant technology in the wireless LAN market. The basic service set (BSS) is the fundamental building block of an IEEE 802.11 LAN. Each BSS consists of at least two stations. Two types of networks are supported: the Infrastructure BSS and the Independent BSS.
In the Infrastructure BSS, stations communicate via a central AP. The central AP receives traffic from a source station and relays it to a destination station. In the Independent BSS, each station communicates with others directly, without the assistance of an AP. Other than traffic forwarding, an AP also provides access to the distribution system (DS), connecting a wireless network with external networks (Ethernet LANs, Internet, etc.). Therefore, the Infrastructure BSS is used to implement enterprise networks that require such connectivity. On the other hand, the Independent BSS can readily be employed to establish an ad hoc network.
A typical Infrastructure BSS is illustrated in FIG. 1, in a BSS of three stations 103 served by an AP 102 that is connected to an Ethernet Switch 101. The wireless stations 103 communicate with one another via the AP 102 and can access resources in a wired external network 100 via the AP 102, which has another interface 101 to the wired external network. In a typical setup, such as illustrated in FIG. 1, the wireless stations 103 can, for example, download files or stream video from the server(s) 100 that are connected to the wired network.
According to the IEEE 802.11 LAN MAC protocol, once a packet has been sent to the wireless media, the sender waits for an acknowledgement from the intended receiver. If the acknowledgement is not received after a certain time has elapsed, i.e., a timeout occurs, the sender assumes that the previous packet did not reach the receiver. The sender retransmits the packet and again waits for an acknowledgement. When no acknowledgement is received, the sender repeats the send-packet/wait-for-acknowledgement procedure until a pre-determined retransmission limit is reached. When this limit is reached, the sender gives up and the transmission is considered to have failed.
A transmission error can occur for a variety of reasons, very often caused by a bad wireless link condition. The link condition can go bad gradually as when a wireless station moves away from an AP, e.g., during handoff. Or, the link condition can go bad all at once, which occurs less commonly, as in the case when the wireless station is terminated abruptly during a transmission. Even when the station is not moving or terminated, the link condition can still fluctuate significantly over a short period of time due to small scale fading of the wireless channel.
In the typical wireless LAN setup, illustrated in FIG. 1, all down stream traffic from the network server(s) 100 to the wireless stations must be forwarded by an AP 102. That is, the AP 102 is responsible for forwarding the traffic from the server(s) 100 to the destination wireless stations and since there is only one wireless media, all traffic addressed to different wireless destination stations must compete for access to the media to reach their destination stations.
If the link between the AP 102 and one of the wireless stations 103 becomes bad, either because the station moves away during handoff or the station is terminated, the AP 102 will not be able to reach the station while trying to deliver a packet. Due to the retransmission policy, however, the AP 102 diligently attempts to deliver the packet and retransmits over and over until a pre-determined retransmission limit is reached. As a result, the effective bandwidth consumed by the bad link increases multi-fold. This increase in bandwidth usage comes at the expense of other traffic whose share of bandwidth decreases correspondingly. The net effect is that the quality of traffic on good links suffers because of the increased traffic on the bad link. Instead of one packet being transmitted, multiple packets are transmitted in a futile attempt to deliver the packet over the bad link.